In vehicles with body over chassis design & using worm-wormwheel steering gear a steering linkage is provided to connect the steering column and the steering gearbox. The purpose of this steering linkage is to transmit the torque applied at the steering wheel to the steering gear thus causing the required directional change of the vehicle. During a front-end collision of a vehicle, deformation occurs in the front end of the vehicle which will be transmuted to the steering column along with steering wheel results into intrusion of sufficient magnitude which can cause fatal injuries to the driver. It is therefore of paramount importance to provide a mechanism which would minimize or eliminate the steering intrusion into the driver's compartment.
U.S. Pat. No. 4,746,144 discloses a breakaway steering assembly constructed such that the steering gear will mechanically separate from the steering shaft to reduce the potential injury to the driver in the event of a front-end collision. Said invention discloses a breakaway steering assembly comprising a hollow tube and a solid shaft which is kept together by means of pins and reinforcement metal band. The deflection member is in the form of a ramp that extends at an acute angle from the wall (18) of the hollow tube (14) into the channel (20). The diameter of the tube is greater than the diameter of the part of channel, which normally receives the shaft. The solid shaft has a projection, which is normally axially located between the metal band and the alignment tabs. A shear pin secures the tube and the shaft together so that the tube and the shaft functions as a unit during normal operations. However, in a collision, the shear pin will fail at a predetermined axial force, causing the hollow tube to move axially until the ramp contacts the angled end of the solid shaft. The movement of the end of the shaft along the ramp begins to drive the shaft out of the tube. This motion makes the metal band to be in tension until it is fractured by the projection and also causes the alignment tabs to bend thus allowing the shaft to leave the tube.
The disadvantage of the above said invention is that since it uses shear pin and reinforcement band, to keep the assembly together, the breaking away of the link requires force more than the reactions generated by the shear pin as well as the reinforcement band during separation. The dimensional variations of the channel and shaft will result into variable breakaway force & therefore would require a selective assembly of these parts, though they are within the specified tolerance band. While adapting this mechanism for use in a vehicle, an additional sliding joint will be required to be put in series to take care of the undulations caused by the vehicle environment. Also, the entire size of the mechanism would be bigger which is not desirable where there is a space constraint. If the tube and shaft separates during handling or during normal vehicle operation, they would require replacement.
The FIGS. 1 to 4 of the enclosed drawings show a prior art steering system. Wherein FIG. 1 shows complete layout of steering system
FIG. 2 shows enlarged view of telescopic joint in the intermediate link
FIG. 3 shows a sectional view of outer tube of the telescopic joint at A-A in FIG. 2
FIG. 4 shows a sectional view of inner shaft of the telescopic joint at A-A in FIG. 2
The steering linkage begins with the steering wheel (1) in the passenger compartment. It is mounted on the steering column (2). The steering column is connected to the steering gearbox (4) through an intermediate link (3). In the event of a crash, the steering gearbox (4) is pushed backwards. The displacement of the steering gearbox (4) is transmitted to the one end of the steering column (2) through a telescopic intermediate link (3) when its telescopic movement is exhausted. One of the ways to restrict the steering column movement is to break the intermediate link (3) to separate its linkage to the steering gearbox (4) under high structural movements, which would occur in the event of a crash.